(1) Neuronal homeostasis depends on normal mitochondrial function to provide 4 the energy to maintain membrane polarization, sequester and extrude calcium, and drive ion pumps. Enzymes of the mitochondrial electron transport (ET) chain are regulated, in part, by the bioenergetics needs of the cell. The relative density of excitatory input to a brain region helps to determine local neuronal activity which, in turn, regulates metabolic demand and, consequently, ET enzyme levels and activity. We hypothesize that by examining the status of enzymes in the proximal and distal portions of the mitochondrial ET chain, it will be possible to assess in vitro one index of regional neuronal activation (recent metabolic demand) in control and Alzheimer's status in AD. The first enzyme in the ET chain, complex I, will be assayed in control and AD brain sections, using quantitative autoradiographic binding of [H]dihydrotenone to label the ND-1 subunit of the enzyme. The terminal ET chain enzyme, complex IV or cytochrome oxidase, will be assayed using quantitative histochemical staining. The quantitative distributions of complex I and complex IV will be compared to determine whether there is relationship changes in AD. Preliminary data indicate that both complexes I and IV are substantially reduced in dentate gyrus in AD. We hypothesize that this decrease reflects a reduced metabolic demand in this region, resulting from loss of perforans pathway input from entorhinal cortex. We will test this hypothesis in rats after unilateral entorhinal cortex lesions by assessing complex I and IV 7 days and 30 days postlesion. Other regions of human brain to be examined in future studies include entorhinal cortex, subiculum, superior temporal gyrus, primary motor cortex, primary sensory cortex, striatum, substantia nigra and cerebellum. (2) Impaired electron transport function predisposes to excitotoxic neuronal degeneration by reducing membrane polization and relieving the Mg** blockade of the NMDA receptor ion channel. In the presence of a bioenergetics defect, normal concentrations of extracellular glutamate may become lethal. There is indirect evidence that excitotoxicity may play a secondary role in neuronal death in AD, and there is in vitro and in vivo evidence for bioenergetics defects in AD. We hypothesize that mismatches between bioenergetics status (ET enzyme activity) and excitotoxic potential (NMDA receptor density) may help to define selectively vulnerable brain regions. For example, CA1 and dentate gyrus have similar high densities of NMDA receptors, but CA1 is selectively vulnerable to excitotoxicity and neurodegenerative disorders, including AD. Our preliminary data indicate the complex I levels and complex IV activity are much lower in CA1 than dentate gyrus. The relative lack of ET enzymes in a region with a high density of NMDA receptors may predispose to neuronal degeneration. We will compare in detail the distributions of complexes I & IV to NMDA receptors in serial sections in multiple regions of human brain.